1. Field of the Invention
This invention is related generally to low sheet resistance vitreous enamel resistors such as are produced with screen printing techniques and subsequent firing.
2. Description of the Related Art
Thick film resistors and conductors most closely related to the type manifested in the present invention are manufactured by screen printing a desired pattern of electrically conductive material onto an electrically non-conductive substrate then firing the substrate at a temperature sufficient to cause bonding to take place within the material and between the material and the substrate. Electrical conduction is then able to occur along the formed pattern.
The conventional approach to formulating a low sheet resistance pattern is to use an alloy of silver and palladium, with a majority palladium. This alloy is then blended with a glass mixture and appropriate screening agents so as to provide screenability and desired resistivity.
While the sheet resistivity may be one ohm/square or less with TCR (Temperature Coefficient of Resistance) values within 100 ppm/.degree.C. over the range of -55.degree. C. to +125.degree. C., the mixture is one which includes a large amount of palladium. Significantly, palladium is presently many times as expensive as base (non-noble) metals. Additionally, long term stability of palladium-silver formulations may be affected by silver migration.
In Howell U.S. Pat. No. 794,518, incorporated herein by reference, discloses an alternative method for formulating a vitreous enamel type electrical resistor. In the Howell patent, an alloy of copper and nickel is milled to a size less than 5 microns, mixed with a glass frit, and subsequently fired in a nitrogen atmosphere. The resulting material is shown in various examples to have a good TCR through the -55.degree. C. to +125.degree. C. temperature range.
While the approach detailed by Howell may be used to provide satisfactory resistors, several deficiencies have been observed. In starting with an alloy, an ink manufacturer is limited to the selection of commercially available alloys, which are far from unlimited in ratio between copper and nickel. The selection of ratio of copper to nickel allows significant design flexibility in customizing the ink to fit an application.
Additionally, the alloy is difficult to mill to a small size as required by the Howell disclosure, requiring a substantial amount of milling time. Impurities may be presented during the milling process which affect the reliability of the finished vitreous enamel resistor. Such impurities may require great expense in a typical manufacturing operation to pinpoint.
In the Howell method, energy is expended to form the alloy, and then again to mill the alloy to size. As in the present invention, the material disclosed by Howell will then require standard firing. This energy and extra processing to form the alloy and mill the alloy adds to the cost of the material. The time needed from order placement to ensure delivery of a finished product (order turn around time) is increased due to the extra processing.